1. Field of the Invention
The present invention relates to a method and an imaging apparatus for correcting a defective pixel of a solid-state image sensor, and particularly to a technique that is applied to an electronic image recording apparatus such as a digital camera or movie camera, which is a signal processing technique for correcting defects (flaws) in photosensitive pixels that may occur in production of a solid-state image sensor. The present invention further relates to a digital camera which comprises an imaging device having two types of light-receiving elements of different light-receiving sensitivities and light-receiving signal saturation levels for each pixel, and a method for creating pixel information in the imaging device mounted on the digital camera.
2. Description of the Related Art
A solid-state image sensor such as a charge-coupled device (CCD), used in a digital camera or the like has a very small dynamic range compared to a general silver halide photograph, and therefore even an image captured through correct exposure may seem somewhat unsatisfactory compared to a silver halide photograph. Moreover, so called loss of shadow detail and burnt highlight may occur, resulting in significant degradation in image quality depending on image-capturing conditions. For eliminating these disadvantages, a method of capturing a plurality of images of different levels of exposure in the same scene and combining the plurality of image data by computation to obtain an image having an increased dynamic range has been proposed.
Japanese Patent Application Publication No. 9-205589 discloses a CCD solid-state imaging apparatus divides one unit cell into two types of light-receiving areas (high sensitivity area and low sensitivity area) having different sensitivities for a large number of light-receiving portions (unit cells) two-dimensionally arranged on a light-receiving surface, and mixes or adds signals read from the two light-receiving areas, respectively, thereby achieving an increase in dynamic range.
The solid-state image sensor of a CCD or the like is produced by forming a large number of photosensitive elements such as photodiodes on a semiconductor substrate but in production of the image sensor, a defective pixel unable to capture a pixel value on a local basis may occur due to impurities introduced in the semiconductor substrate or the like.
For the image sensor having such a defective pixel, Japanese Patent Application Publication No. 7-143403 discloses a technique of correcting the pixel value of the defective pixel according to combined signals from a plurality of surrounding pixels adjacent to the defective pixel.
An imaging device using a CCD or the like has its light-receiving area formed by integrating many hundred of thousands to millions of very small light-receiving elements, and it is therefore difficult to produce an imaging device having no defective light-receiving element, i.e., defective pixel. Thus, correction processing is carried out such that existence/nonexistence of a defect for each pixel (defect data) of the imaging device is acquired in advance, a data table indicating whether or not each pixel is defective, the data table is recorded in a nonvolatile memory of a digital camera equipped with the corresponding imaging device, and the digital camera replaces a signal of each defective pixel with a signal of an adjacent normal pixel with reference to the data table recorded in the nonvolatile memory when an imaging signal is processed (see Japanese Patent Publication No. 1-29475, for example).
In a method for correcting flaws of a solid-state image sensor, which has been conventionally practiced, correction is carried out such that a photosensitive pixel exhibiting an irregular behavior equal to or greater than a defined level in a production process is detected as a flaw, and if the number of flaws is equal to or less than a defined number, signals outputted from photosensitive pixels determined as flaws are constantly replaced with surrounding pixel information, or an average value of several surrounding pixels is constantly outputted.
According to this conventional correction method, if a considerable number of photosensitive pixels are determined as flaws, and signals outputted from these flaws are corrected with the values of surrounding pixels, resulting image data would be as if filtered by a lowpass filter (LPF) on a local basis even before image signal processing.
A wide dynamic range imaging device having two types of light-receiving elements of different light-receiving sensitivities and light-receiving signal saturation levels for each pixel has essentially an increased number of light-receiving elements. In this case, if a data table created for each of accumulated charges of light-receiving elements associated with low luminance (standard signals), accumulated charges of light-receiving elements associated with high luminance (high luminance signals) and mixed outputs of standard signals and high luminance signals is recorded in a nonvolatile memory, the nonvolatile memory requires having a large recording capacity, and correction processing is complicated.